BARCELONA, Spain (AP) — Sebastian Vettel's poor form continued at the Spanish Grand Prix on Sunday.

Vettel finished fourth at the Circuit de Barcelona-Catalunya, extending his winless run to three races and seeing Lewis Hamilton increase his lead to 17 points in the drivers' championship.

"Today we just didn't have the right pace, Mercedes was faster than us and we hadn't many chances," Vettel said. "We tried our best, but something was missing and we need to improve on this."

Vettel got off to a great start this season, winning the first two races, before his fortunes changed for the worse.

Vettel was also fourth in Azerbaijan, when he made a mistake while trying to pass Valtteri Bottas late in the race. In China, he couldn't manage more than eighth place after an ill-timed safety car and a subsequent collision caused by Max Verstappen.

He had won in Bahrain and in the season-opener in Australia, when he wasn't as dominant as Hamilton but took advantage of a safety car to win on race strategy.

Vettel started well in Spain, making an outside pass on Bottas on the first turn. He wasn't fast enough to keep pace with Hamilton up front but looked quick enough to at least secure second place.

But Vettel struggled with tire wear and Ferrari decided to bring him in for an extra stop. The team hoped Vettel was going to be faster than his opponents afterward, but it didn't work that way and he never challenged Verstappen for the final podium spot.

"Our tires didn't last as long as for the others, so we couldn't follow the same strategy," Vettel said. "We had to stop again and we obviously lost two positions, and also a bit of time during the pit stop itself. But staying out was not an option today. We had problems finding the right balance of the car and we were struggling with the front tires."

Vettel said he remained upbeat heading into the Monaco Grand Prix in two weeks.

"During the whole weekend it has been tough to find the right balance on these new tires, but I see no reason for not being optimistic for the next race," Vettel said. "I am happy to go back to 'normal' tires in Monaco as we seemed to struggle a little bit more than the others."

Vettel's teammate, Kimi Raikkonen, also struggled in the Spanish GP, having to retire because of a loss of power near the halfway-point.

"This result is far from ideal, and I'm pretty disappointed," said Raikkonen, who is fourth in the drivers' standings, behind Bottas. "There is nothing we could have done differently today, but we need to try and keep pushing."

The poor results by Vettel and Raikkonen dropped Ferrari behind Mercedes at the top of the constructors' championship.

"In many ways, this is a weekend in which nothing went right," team principal Maurizio Arrivabene said. "Calmly, we need to analyze the reasons, and they are various, which prevented us from exploiting the potential we demonstrated in the previous races."